ing the tint that is suited to them, and present very different aspects according to the mode in which they are lighted. A red ribbon, for example, placed successively in the different colors of the spectrum, appears black, except in the red region; it therefore returns by reflection an almost homogeneous light. A rose ribbon appears very unequally luminous in different parts of the spectrum. The light which it reflects is, therefore, complex.
We may ask, then, What would be the condition of nature if the light that shines upon us were absolutely homogeneous? Some bodies would absorb it completely, and would appear dark like black velvet; others would reflect it more or less actively, and would have a corresponding degree of brilliancy. As there would be no criterion for comparison, the eye would have only the sensations of white, black, and the intermediate rays.
Pascal said that nothing better enabled him to comprehend the properties of the air than what took place where there was none. So, nothing better enables us to comprehend the properties of colors than the appearance of the world under an illumination of homogeneous light. The volatilization of a salt of soda in the flame of a Bunsen burner almost perfectly fulfills this condition. With such a light, cloths dyed in the richest colors show only white, black, and gray, and the art of painting has no place.
The estimation of color being connected with the impression produced upon the retina, it is readily to be seen that the human eye will not always equally well perform that function. The different points of the retina are not alike ready to appreciate colors. To distinguish the details of au object, it is necessary to direct the look toward it, or, in other words, to produce an image upon the central region of the retina, where the acuteness of physiological perception is much the greatest. The same is the case for colors. When we keep the look in a determined direction, and put a colored body in the visual field in such a way that its image is produced laterally, we remark that the notion of color is more and more weakened as we remove from central vision, and disappears at the limits of the field. But the most important fact is, that in the different views the colors are not distinguished from one another with equal facility, and that we sometimes come to the point of confounding colors which really seem to be most discordant, as green and red. The discovery of this particular form of infirmity is due to Dalton, who was very strongly affected by it, and who carefully analyzed the errors of his judgment. This fault, which remained unperceived for so long a time, is in reality quite frequent. About ten persons in a hundred make mistakes in the comparison of colors marked enough to be detected by an attentive examination. Generally the imperfection is not accompanied with grave inconveniences, and is corrected unconsciously by the operation of habit, the recollection of objects, and the judgments of others. But the annoy-
